The present invention relates to electro-hydraulic braking EHB systems for motor vehiclesxe2x80x9d.
Electro-hydraulic braking systems for motor vehicles are known which comprise a brake pedal, a braking device connected to at least one vehicle wheel, which is capable of being brought into communication with an electronically controlled valve arrangement in order to apply hydraulic fluid under pressure to the braking device, a hydraulic pump, and a hydraulic pressure reservoir fed by said pump for the provision of hydraulic fluid under pressure which can be passed to the braking device via the electronically controlled valve arrangement in order to apply hydraulic fluid under pressure, in proportion to the driver""s braking demand as sensed at the brake pedal, to the braking device in so called xe2x80x98brake by wirexe2x80x99 mode.
The area of application of an electro-hydraulic braking system of this form encompasses among others anti-locking brake system control, anti-slip control, electronic brake force distribution, and vehicle stability control VSC.
In view of the act that, with an electrically-actuated braking system, the driver""s wishes are acquired by sensors at the brake pedal, and conducted to the electronic control system by means of electrical signals, such systems are described as electronic braking systems (EBS) or xe2x80x9cbrake-by-wirexe2x80x9d systems.
Furthermore, in modern vehicle systems, there are requirements to reduce costs, component weight, complexity and, more recently, to reduce system energy consumption. These savings have to be made without detriment to safety or system function. In the case of an electronically controlled braking system (EBS or Brake by Wire) having automatic hydraulic or pneumatic (fluid) braking, savings can be made in all areas by addressing the issue of pressurised fluid storage for brake actuation.
In a typical EBS system, a fluid pressure accumulator (pressurised reservoir) is used to store fluid under pressure, charged by use of a fluid pump or compressor driven either by an electric motor or directly from the engine via a belt and pulley arrangement, for supply to a valve system which is adapted to supply the brake or brakes of a vehicle with fluid pressure in accordance with the level of driver""s braking demand. Typically, the pressure at which the fluid is stored within the accumulator is set at a relatively high level, which is set so as to be sufficient to actuate the brakes of the vehicle over the complete braking range of the vehicle. In order to achieve this relatively high pressure storage, a high pressure accumulator and pump system has to be employed with sufficient volume to supply several low pressure demands as well as sufficient pressure and volume to supply the infrequent high pressure brake applications.
It is common practice for the fluid pressure accumulator to be of the gas-charged diaphragm type wherein the housing of the accumulator is divided into two parts by means of a flexible diaphragm, one side of the diaphragm being coupled to the fluid pump and to pipework connected to the electronically controlled valve arrangement and the other side being closed and pre-charged with a volume of gas.
In accordance with conventional practice, the control of the accumulator pressure is arranged to lie within a fixed control range, with a lower limiting pressure of the control range being the so-called xe2x80x9ccut-inxe2x80x9dpressure and an upper limiting pressure of the control range being the so-called xe2x80x9ccut-outxe2x80x9d pressure. For this purpose, the accumulator pressure is sensed by means of a pressure sensor and evaluated by means of an electronic control unit which controls the accumulator by means of the aforementioned pump such that the pump is started when the accumulator pressure tends to fall short of the cut-in pressure level and is stopped when the accumulator pressure reaches the cut-out pressure level.
Below the cut-in pressure there is defined a so-called xe2x80x9cwarning pressurexe2x80x9d level whereby there is a warning pressure range defined between the warning pressure level and the cut-in pressure. Thus, at times, in particular before starting the pump, the accumulator pressure can be in the warning range. However, if the accumulator pressure should fall short of the warning pressure level, then the electronic unit can draw conclusions in connection with the accumulator state as well as system faults and act accordingly, for example to energise a warning lamp.
It is known already that such an accumulator has the disadvantage that the pre-charge pressure of the gas depends on the temperature. This means that the lower is the temperature, the lower is the pre-charge pressure and vice versa. Consequently, the fluid volume available in the control and warning range also depends on the temperature, namely the higher is the temperature, the greater is the fluid volume available. Reference is directed to FIG. 1 of the accompanying drawings in this connection, which illustrate the latter coherence.
In safety critical applications, such as electro-hydraulic braking systems (EHB), variations of the available fluid volume are undesirable. In particular, in electro-hydraulic braking systems a minimum fluid volume should always be provided in the warning range so that a prescribed number of brake actuations is always capable of being executed, Besides, variations of the available fluid volume cause disturbing noises since they lead to a high operating frequency of the pump.
In accordance with the present invention, at least one of the warning, cut-in and cut-out pressure levels is arranged to be controlled in dependence upon temperature so that the available fluid volume in at least one of the control and warning ranges is maximised.
In one preferred embodiment, all three of the warning, cut-in and cut-out pressure levels are arranged to be varied with temperature.
In another preferred embodiment, the warning and cut-in pressure levels are arranged to be varied with temperature but the cut-out pressure remains constant with varying temperature.
In all cases, the variations must be coordinated so that the characteristic curves do not cross within the working ranges.
Advantageously, the pressure levels fall with temperature up to a predetermined temperature and then rise with temperature beyond that temperature.